With the development of urbanization, the wastewater treatment rate is increasing. The sludge produced in the wastewater treatment is continuously increased, and therefore the treatment and disposal of the sludge becomes an intractable problem. In the wastewater treatment plant, a process of concentration, digestion and dewatering is usually used, the residual sludge produced in the wastewater treatment is treated in the plant to a dewatered sludge having a water content of 70-85%, and then the dewatered sludge is moved out of the plant for a further treatment or disposal. Since the dewatered sludge has a high water content and is in a high mass and volume and in a semi-solid state, it is not favorable for its loading and unloading, transport and storage. In addition, the dewatered sludge cannot be directly subjected to the treatment such as landfill, incineration, producing the building material and producing the fertilizer, and has to be further dewatered for the further disposal. Therefore, the further reduction of the water content of the dewatered sludge becomes an essential stage of the sludge disposal.
In the wastewater treatment plant, it is usually to add a polyacrylamide solution to the residual sludge or a concentrated sludge to conduct a flocculation conditioning, then the conditioned sludge is subjected to a solid-liquid separation with a centrifuge, a plate and frame filter press or a belt filter press to remove a part of water in the sludge to produce a dewatered sludge having a water content of 70-85%
In the above sludge dewatering process, the sludge particles are flocculated under the effect of the electrostatic neutralization and adsorption-bridge of the high molecular flocculant polyacrylamide to form larger sludge particles that enwrap a large quality of water. The enwrapped particles bond more firmly under the mechanical solid-liquid separation of the dewatering machine to form a strong sludge aggregation structure. This results in that the dewatered sludge having a water content of 70-85% is in a viscous semi-solid state, which brings great troubles to the subsequent further dewatering and drying.
The troubles are in the following aspects:
(1) In case of the further dewatering by conditioning, solid or liquid conditioning agents should be directly added to this semi-solid dewatered sludge. However, it is difficult to stir the resulting mixture homogenously. Therefore the conditioning effect will be adversely influenced;
(2) This dewatered sludge has a high viscosity and is apt to bond or block-up the device in the drying process, causing the device failure and increasing the risk of safety accident;
(3) This dewatered sludge is in a viscous state. Even if it is mechanically pre-dispersed, it will quickly bond together in the drying process, which reduces the sludges specific surface area, and influences the heat transfer efficiency for the heat-drying. It is easy to reduce the water content in the sludge's surface, but it is difficult to reduce the water content in the sludge's interior. If the water content of the whole sludge is to he reduced, the heat quantity for drying and the air quantity should be increased or the residual time of the sludge should be prolonged, which results in a higher energy consumption as well as an uneven drying (the exterior is dried while the interior is still wet, the so-called “soft yolk” phenomenon).
There are mainly two methods of retreating the dewatered sludge of this kind to farther decrease its water content: one is the heat drying method and the other is the lime stabilization method. For the heat drying method, water in the dewatered sludge is usually removed by the heat convection or the heat conduction so as to reduce the sludge's water content. The heat drying method can accomplish the sludge minimization by reducing the sludges water content. If a dewatered sludge having a water content of about 80% is dried to one having a water content of about 30%, the volume of the dewatered sludge can be reduced to 28% of its original volume, and therefore the mass and volume of the sludge to be subsequently disposed can be sharply decreased, and the cost for the subsequent sludge disposal can be remarkably reduced. However, it is difficult to disperse this viscous dewatered sludge, and therefore the dewatered sludge has a smaller specific surface area, the efficiencies of the mass transfer and the heat transfer upon drying are lower, and a large quantity of heat has to be consumed to evaporate water. Moreover, a large quantity of waste gas that is unmanageable will be produced in the drying process, and therefore the operation cost and the waste gas treatment cost are high. For the lime stabilization method, calcined lime is added to the dewatered sludge. The water content of the sludge is reduced by increasing the mass of the sludge on the dry basis, and the water content of the dewatered sludge is also decreased by evaporating a small amount of water in the sludge with the exothermal reaction of calcined lime. However, the lime stabilization will increase the mass of the treated sludge, rather than reducing the mass of the sludge. Therefore, the subsequent disposal cost will be increased. Moreover, the lime stabilization method produces a large quantity of ammonia, and therefore the cost for the waste gas management will increase.
The lime stabilization method is a conventional technology for treating sludge. JP 3083880A discloses a process of reusing sludge, in which calcined lime is used to remove water in sludge and dry the sludge. Specifically, the sludge and calcined lime are mixed and stirred in a stirrer having a specific structure to remove water in the sludge, and the produced heat is utilized to sterilize the sludge. The small dried sludge particles are processed into tine powder with a sludge powder-particle separator, and the resulting fine powder can be used as fertilizer or snow-melting agent.
The method disclosed in JP 3083880A utilizes the lime stabilization technology and represents the characteristic of the Hine stabilization. That is to say, a large amount of calcined lime is added. Because calcined time is used to react with water in the sludge to dry the sludge and the sludge has a higher water content, a large amount of calcined lime is added, resulting in a high operation cost. In the meanwhile, because a large quantity of lime is reacted with water in the sludge, the amount of the dry sludge is increased by comparison with that before the treatment, and usually increased by 50-100% of the dewatered sludge. This is opposite to the object of minimization for the retreatment of the dewatered sludge. In addition, JP 3083880A aims at a special sludge, river sludge, which has a more amount of mud and sands and a less amount of organic substances. Therefore, a less amount of ammonia will be produced in the reaction of the sludge and calcined lime. On the contrary, the dewatered sludge from a common wastewater treatment plant has a more amount of organic substances, and therefore a more amount of ammonia will be produced in the reaction of the sludge and calcined lime. This results in a bad operation environment and a safety risk. Moreover, the subsequent waste gas treatment is complex and costly.
CN101823825A discloses a process and an apparatus for dewatering and drying the sludge, wherein one or more of fly ash, fine manganese slag, calcined lime, ferric chloride; solution, and aluminum sulphate solution (preferably 50 kg of calcined lime and 2 kg of ferric chloride solution per one ton of the sludge) is added to a residual concentrated sludge; and the sludge is dewatered so that the water content of the sludge is below 60%; the dewatered sludge is sent to a pre-drying zone, a certain amount of calcined lime, potassium permanganate and a finished dry sludge is also sent to the pre-drying zone, and the resulting mixture is mixed homogenously; the pre-dried sludge is sent to a hollow paddle indirect drying machine; and the finished dried sludge is obtained after drying, wherein a part thereof is sent back to the pre-drying zone.
CN101823825A aims at the residual concentrated sludge having a high water content, usually 99-97%, which is in a liquid state and is not modified by adding polyacrylamide and therefore has no aggregation structure. From the viewpoint of the sludge's structure, ifs easier for conditioning and dewatering. This patent application has no technical solution solving the problems such as the aggregation of the dewatered sludge caused by adding polyacrylamide which lead to a high energy consumption for drying, an uneven drying, and viscously bonding and blocking-up the device. This patent application is silent about dewatering the residual concentrated sludge having a water content of 99-97% to the level of 60%. This patent application teaches that at least two substances, an inorganic agent and the dried sludge are added to the dewatered sludge having a water content of below 60%, where the addition of a strong inorganic oxidant is for a better digestion. This patent application teaches that pile-turning is conducted for an aerobic digestion to reduce the organic substances in the sludge, which is advantageous for subsequently producing the budding material. It can be seen that the object of Conditioning the dewatered sludge and back-mixing the dried sludge is for a better digestion, rather than for decreasing the water content.
GB2431924A discloses a process and an apparatus for treating a liquid sewage sludge having a water content of 90% or more. A dry lime is added to a buffer zone and a high strength mixing zone of a contacting trough to increase the pH value of the liquid sewage sludge. The sludge and lime are recovered by a pump from the two zones. The treated sludge is they subjected to a dewatering process. The process and the apparatus in this patent application aim at a liquid sludge having a water content of 90% or more. The object is to improve the dewatering property of the liquid sludge before dewatering the sludge. This patent application is not suitable for the dewatered sludge having the water content of 70-85% and cannot accomplish the further dewatering of the dewatered sludge of this kind. Since the sludge having a water content of 70-85% is in a semi-solid state, it is substantially different from the liquid sludge. The apparatus of this patent application fails to accomplish mixing the dewatered sludge and lime homogenously. The addition of lime to the dewatered sludge is the so-called lime stabilization technology, the disadvantage of which has been discussed hereinbefore.
CN101224912A discloses a process for drying the sludge, which comprises: drying a dewatered sludge (having a water content of 69-51%) to the level of 10-1% as sludge drying support; conveying the sludge drying support and the dewatered sludge to be dried respectively to a stirring and mixing apparatus, stirring and mixing the resulting mixture therein to produce a mixed sludge (having a water content of 35-25%), using a part of the mixed sludge as starting material for producing the drying support and the other mixed sludge as the finished product, and conveying the mixed sludge which is used as starting material for producing the drying support to the drying apparatus to conduct the drying.
The method disclosed in CN101224912A aims at the dewatered sludge having a water content of 69-51%. For the dewatered sludge from the wastewater treatment plant having a higher water content (70-85%), the amount of the drying support to be back-mixed will be greatly increased, and therefore the corresponding load for the mixing and drying devices will be increased correspondingly. The method disclosed in CN101224912A is a technology in which the sludge is back-mixed. Although it can avoid the uneven drying of the sludge and can form good sludge particles, increase the sludge's specific surface area, improve the efficiencies of the mass transfer and the heat transfer, and has it lower operation energy consumption, it cannot overcome the problems that the dewatered sludge produces an aggregation structure and enwraps a large quantity of water caused by using polyacrylamide for conditioning in the wastewater treatment plant, and the semi-solid state of the dewatered sludge brings about the viscosity. Furthermore, the high speed mixing (where the linear speed is 100-527 m/min, and the angular speed is 55-280 r/min) in its stirring and mixing device cannot accomplish the pro-dispersion of the viscous sludge.
As set forth above, a large quantity of the dewatered sludge having a water content of about 70-85% obtained from the wastewater treatment plant is in demand of further reducing its water content for the subsequent treatment and disposal. The methods in the prior art have the following disadvantages: (1) The sludge to be treated is a sludge that is in a liquid state and is not modified with polyacrylamide, or a sludge having a lower water content, or a river sludge having a less amount of organic substances, and is different from the semi-solid dewatered Sludge having a water content of 70-85% that is obtained from the wastewater treatment plant and is dewatered after adding polyacrylamide. Therefore it is not applicable for the methods in the prior art. (2) The lime stabilization technology is generally used to treat the dewatered sludge. However, the addition of a large quantity of lime to the sludge does not conform to the principle of minimization, but increases the amount of the sludge to be subsequently transported, treated and disposed and increases the costs for the treatment and the disposal. Furthermore, a large amount of ammonia will be produced in the mixing, which increases the cost for treating the waste gas. (3) Adding the dried sludge, calcined lime, potassium permanganate and other agents to the sludge and then aerobically pile-turning the resulting mixture can reduce the water content and decrease the amount of the organic substances in the sludge. However, it is not in favor of the subsequent disposal of the sludge for producing the fertilizer. The disposal route is limited. The increase of the agents in kind and amount certainly increases the disposal cost. (4) In case of using the direct sludge drying technology to pass through the high viscosity phase of the sludge (wherein the water content is 60-45%), the energy consumption is high, and the soft yolk phenomenon (the exterior is dried while the interior is still wet) will appear; it is inevitable that the small molecular organic substances will be cracked in the drying; and the treatment of the produced waste gas is difficult and costly. (5) The sludge back-mixing and drying technology can only avoid the uneven drying of the sludge, but cannot destroy the aggregation structure of the sludge formed by the dewatering after the conditioning with polyacrylamide; it is difficult to effectively accomplish the pre-dispersion of the viscous sludge, resulting in that the energy consumption for the subsequent drying will increase correspondingly. Therefore, in order to overcome the above-mentioned problems, there is an urgent demand to develop a new method of retreating a dewatered sludge to treat this special dewatered sludge and accomplish its minimization, innocuity and recycle.